474 
LIFE-HISTORIES 
chain somewhat larger cells are formed, each with a distinct 
wall. Certain of these cells, called heterocysts 1 have most of 
their protoplasm replaced by a pale or colorless cell-sap and 
Fic. 
306.—Fallen Stars (Nostoc spp., Fallen Star Family, Nostocacee). 
A-F, Nostoc paludosum. <A, chain-colony with resting spores near 
the middle and heterocysts at the ends, 23%. B, resting spores. C— 
F, germination of resting spore, and stages in the formation of a new 
chain-colony. G-—J, Nostoc sphericum. G, gelatinous balls as they 
appear in moist earth, 2? natural size. H, part of a slice cut through 
the ball, +92. J, early stages in a growing colony, 222. (Janczewski, 
Thuret.)—Protoplasts bluish green, enveloped in a gelatinous mass 
which they produce. Found mostly in water or on moist earth in 
various parts of the world. 
become incapable of further development. At points where 
the heterocysts thus limit growth the chain eventually 
1 Het’er-o-cyst < Gr. heteros, different; kystis, a bag. 
